Engine air/fuel control systems are known in which fuel delivered to the engine is adjusted by output signals derived from exhaust gas oxygen sensors positioned both upstream and downstream of the catalytic converter. Each output signal is generated by comparing the voltage output of each sensor against a reference to generate a two-state signal.
It is also known to monitor the converter's efficiency by calculating a ratio in transitions of the downstream two-state signal to the upstream two-state signal. Such an approach is shown in U.S. Pat. No. 5,385,016.
The inventors herein have recognized numerous problems with the above approaches. For example, poor engine calibration, or peculiar mixing of the exhaust gases in the exhaust manifold, may cause the downstream sensor to be exposed to exhaust gases rich of stoichiometry. A comparison of the downstream sensor output to a reference voltage may not always be possible. Under such conditions, a transition in the sensor output may not be detected thereby reducing the accuracy of converter efficiency indications. Similar problems may occur when the peak-to-peak voltage output of the sensors becomes degraded.